The present disclosure relates to semiconductor devices, and particularly to vertical polysilicon-germanium heterojunction bipolar transistors and methods of manufacturing the same.
The collector current of a bipolar transistor is a function of the energy bandgap of the base region material. For a regular silicon base bipolar transistor, it takes an emitter-base bias of about 0.92 V to obtain a collector current density of 10 mA/μm2. It is desirable to be able to reduce the voltage needed to drive a bipolar transistor.
The state of art SiGe-base bipolar transistors have silicon in the collector, a SiGe alloy in the base, and polysilicon as emitter. The distribution of Ge across the SiGe base region can be relatively uniform, or can be graded with higher Ge concentration near the collector end and lower, or zero, Ge concentration at the emitter end.
An example of the graded Ge distribution across the base region with zero Ge concentration at the emitter end can be found in the paper by D. L. Harame et al., “Si/SiGe epitaxial-base transistors—Part I: materials, physics, and circuits,” IEEE Transactions on Electron Devices, vol. 42, p. 455, 1995. The SiGe base region is single crystalline, and is grown epitaxially on top of a single crystalline silicon collector layer. With zero or relatively low Ge concentration at the emitter end, there is little energy bandgap difference between the emitter and the base at the emitter-base junction. Thus, this transistor is not a wide-gap-emitter heterojunction bipolar transistor.
An example of a SiGe-base bipolar transistor having relatively uniform Ge distribution across the base region can be found in the paper by P. Deixler et al., “Explorations for high performance SiGe-heterojunction bipolar transistor integration,” Proceedings of BCTM, p. 30, 2001. The SiGe base region is single crystalline, and is grown epitaxially on top of a single crystalline silicon collector layer. With the Ge distribution that is relatively uniform across the base, the energy bandgap of the base region is smaller than the energy bandgap of the emitter at the emitter-base junction. While this is a wide-gap-emitter heterojunction bipolar transistor, the Ge concentration in this prior art is only 15%, resulting in an energy bandgap of about 1.0 eV. Further increase in the Ge concentration in the SiGe alloy in the base region is not possible because Ge concentration greater than 15% in a SiGe alloy destroys epitaxial alignment between the base and the collector due to excessive lattice mismatch between the underlying silicon material and the SiGe alloy. In other words, to avoid defects being generated in the SiGe-base region, the Ge concentration has to stay below some limit, i.e., at about 15% Ge or less.